James Landale, diplomatic correspondent, BBC News, writes on this custom from the earliest days of diplomacy to the present

Diplomats come in many forms, but few are penniless former Wimbledon tennis champions. Boris Becker, the one-time German sporting prodigy, currently glories in the title of Attaché to the European Union for Sporting, Cultural and Humanitarian Affairs on behalf of the Central African Republic (CAR). It is a diplomatic mission Mr Becker has held since April 2018, ten months after he was declared bankrupt. He apparently has an office at the CAR embassy in Brussels. It is not clear whether he has ever been to the impoverished African nation.

But Mr Becker’s diplomatic status is such that he is claiming it gives him immunity from bankruptcy proceedings at the High Court in London. A bank called Arbuthnot Latham and Co is taking legal action against him to try to claim back a large sum of money that it says he owes – reportedly millions of pounds.

The Vienna Convention on Diplomatic Relations 1961 says a diplomatic agent enjoys immunity from a receiving country’s civil and administrative jurisdiction. Mr Becker’s lawyers claim this means that he can be subject to legal proceedings only with the consent of the government of the Central African Republic. The question for the courts will be whether it considers Mr Becker to be a genuine diplomatic agent with legitimate rights of immunity or a retired sportsman using an unusual appointment with an unstable African country as a ruse to avoid paying his debts.

Diplomatic immunity was certainly not invented to sidestep bankruptcy laws. There is little consensus about where the practice first emerged: like all good ideas, it has many parents. But the principle that messengers should be given safe-conduct and not killed or eaten developed in many pre-literate societies. Some point to the ancient civilisations of India,Chinaand Egypt, some to the practices of aboriginal Australia, others to the Greek heralds who were kept safe as they travelled between the city states.

Genghis Khan may have been responsible for the deaths of thousands of people in his time, but he was also very keen on protecting his Mongol ambassadors. He would on occasion wreak devastating vengeance on those who mistreated his envoys. In the early 1200s, Khan invaded and destroyed an entire empire south west of the Caspian Sea simply because a Mongol caravan and its ambassadors had been massacred in one of its cities.

Religious rules, custom and practice, and the idea of reciprocity slowly ensured that immunity became codified in law. Roman ambassadors had the full protection of Roman law.

But it was not until 1708 that the UKparliament formally recognised diplomatic immunity and banned the arrest of foreign officials. The Diplomatic Privileges Act – also known as the Act of Anne – was passed because of an incident involving the Russian Ambassador at the time. Andrej Matveyev was arrested by bailiffs in London because he owed money. He was clearly given a bit of a going over and he protested vehemently against his treatment. Despite apologies from the Queen and parliament, MPs also felt they probably ought to pass a bit of legislation too to assuage the anger of Peter the Great. Thus, diplomatic immunity first entered the British statute book.

The large increase in the number of states in the twentieth century and the corresponding increase in the number of diplomats led to the need for some kind of international agreement about immunity. And that led – eventually – to the Vienna Convention on Diplomatic Relations being agreed in 1961. Article 29 says: “The person of a diplomatic agent shall be inviolable. He shall not be liable to any form of arrest or detention. The receiving State shall treat him with due respect and shall take all appropriate steps to prevent any attack on his person, freedom or dignity.” Article 31 says: “A diplomatic agent shall enjoy immunity from the criminal jurisdiction of the receiving State. He shall also enjoy immunity from its civil and administrative jurisdiction.”

In truth, there are few breaches of immunity. According to the most recent UK government figures, there are about 22,500 people in the UK entitled to some form of diplomatic immunity. Yet in 2016 the police informed the Foreign and Commonwealth Office of only 12 serious or significant offences committed in the UK by people entitled to diplomatic protection. Eight offences involved drink driving.

But if a diplomat is guilty of an egregious breach of the law, there are some things that a country can do.

This is what the Foreign Secretary Boris Johnson said in a written Parliamentary answer in October 2017: “Under the Vienna Convention on Diplomatic Relations 1961, those entitled to immunity are expected to obey the law. The FCO does not tolerate foreign diplomats breaking the law. We take all allegations of illegal activity seriously. When instances of alleged criminal conduct are brought to our attention by the police, we ask the relevant foreign government to waive diplomatic immunity where appropriate. For the most serious offences, and when a relevant waiver has not been granted, we seek the immediate withdrawal of the diplomat.”

Yet around the world there are more and more cases of diplomats pushing the immunity rules to the limit to try to avoid an increasing number of illegal acts. So much so that there are some who believe the Vienna Convention should be reformed and restored to its original intentions.

This, however, is unlikely. At a time when the world is so divided on so many important issues, the chances of achieving a consensus on a lesser issue such as this are small. And at a time when the international rules-based order is so threatened, few countries would want to risk unstitching one of the customs that has underpinned that order from the earliest days of diplomacy.